In an exhaust gas recirculation system, exhaust gas is mixed with the air-fuel mixture of an internal combustion engine. Up to a certain point, an increasing residual gas component in the air-fuel mixture can have a positive effect on energy conversion and therefore on fuel consumption. By increasing the residual gas component, the engine can be dethrottled. In this way, losses due to charge changes are reduced and efficiency is increased. In addition, an increase in the residual gas component results in a reduction in the peak temperature of the combustion process and consequently in a reduction in the formation of nitrogen oxide in the exhaust gas. Increased nitrogen oxide emissions in the exhaust gas occur predominantly in stratified engine operation at an air-fuel ratio xcex greater than 1 in SI engines having direct fuel injection. To counteract this, in an engine having direct fuel injection, exhaust gas recirculation is vital. In this context, exhaust gas recirculation denotes a system in which an exhaust gas mass flow is drawn from the exhaust gas line of the engine and is then conveyed in a dosed manner to an exhaust gas recirculation line leading to the engine via an exhaust gas recirculation valve.
It is unavoidable that in an exhaust gas recirculation system deposits of solid material from the exhaust gas over time can clog the exhaust gas recirculation valve and the recirculation line, the volume of recirculated exhaust gas thus being reduced as the running time of the internal combustion engine increases. For this reason, the functioning of the exhaust gas recirculation system of an internal combustion engine is monitored and measures are taken which compensate for such abnormal changes in the gas flow passing through the exhaust gas recirculation line. From German Published Patent Application No.197 19 278, a method is known which makes it possible to ascertain abnormal changes in the gas flow passing through an exhaust gas recirculation line. In this context, the lambda-control factor is calculated in a switched-on and switched-off exhaust gas recirculation system and the difference between the two control factors is compared to preestablished threshold values with regard to a positive and a negative deviation. When the difference between the lambda-control factors exceeds a threshold value, a fault in the exhaust gas recirculation system is signaled. According to this related art, a fault in the exhaust gas recirculation system is counteracted by using the difference between the lambda-control factors, in switched-on and switched-off exhaust gas recirculation systems, as a correcting quantity, e.g., for the injection time.
The present invention is based on the objective of indicating a method which can very precisely diagnose and compensate for a changed gas flow passing through an exhaust gas recirculation line.
According to the present invention, as a result of the fact that the mass flow passing through the exhaust gas recirculation line, in a first method, is derived as a function of the position and the flow characteristic of an exhaust gas recirculation valve, the mass flow passing through the exhaust gas recirculation line, in a second method, is derived from the fresh-air mass flow in the induction pipe and from the induction pipe pressure, the difference is determined between the mass flows that were ascertained in accordance with the two methods, and then, from the difference between the two mass flows, one or a plurality of correcting quantities is generated for the mass flow which was derived as a function of the valve position and the flow characteristic.
Using the method according to the present invention, it is possible to reliably detect an insufficient exhaust gas recirculation rate at which the exhaust gas threshold values prescribed by law can no longer be maintained. In addition, measures are indicated which make it possible, in a corrective manner, to take into account the detected changes in the gas flow in calculating the mass flow passing through the exhaust gas recirculation line. Using this corrected mass flow, the engine control system can counteract the exhaust gas threshold values are exceeded.
As a correcting quantity for the mass flow, it is possible to use, e.g., an offset for the flow characteristic or a factor that changes the slope in the flow characteristic. A further advantageous correcting quantity is a value that reduces the exhaust gas pressure as measured upstream of the exhaust gas recirculation valve, the value being the product of the square of the mass flow, as determined in accordance with the first method, and the difference between the two mass flows. The choice among the aforementioned correcting quantities is a function of the size of the mass flow, the offset being used correctively in response to a mass flow that is smaller than the slope change, and the reduction in the exhaust gas pressure being used in response to a mass flow that is greater than the slope change.
It is advantageous that a fault in the exhaust gas recirculation system is signaled if the difference between the mass flows determined by the two methods exceeds a preestablished threshold value.
In the event of a fault in the exhaust gas recirculation system, using the measures of the present invention, a correcting quantity is generated which exercises a direct influence on the control system of the exhaust gas recirculation valve. In contrast, in the related art (German Published Patent Application No. 197 19 278), if a fault in the exhaust gas recirculation system has been detected, other operating quantities of the engine, namely the load signal and the ignition, are corrected, i.e., quantities which do not directly relate to the exhaust gas recirculation system, the actual source of the fault.